1. Field of the Invention
The present invention relates to a method of cold rolling steel capable of preventing generation of edge drop by using tandem cold rolling mill stands.
2. Related Art Statement
Hitherto, it has been known that a rapid reduction in thickness (hereinafter called "edge drop") can take place in two widthwise ends of a steel strip during the process of cold-rolling the steel strip.
This problem can occur due to an axial directional metal flow generated at the two side end portions of the steel strip and the oblate surface of the work roll which comes in contact with the steel strip. As described, it depends upon the cold rolling conditions.
For overcoming the problem of the edge drop, there has been available in the prior art a method which involves tapering end portions of work rolls for so-called single-end-tapered work rolls disposed vertically and positioned at the two end portions of a steel strip, in order that the geometrical shape of the roll gap is improved.
Another method has been disclosed in Japanese Patent Publication No. 2-4364 in which the rolling mill including the above-described single-end-tapered work rolls is mounted on at least the first stand of the tandem cold rolling mill stands and a plain work roll is as well as mounted on at least the final stand on the outlet side.
In addition, another method has been disclosed in Japanese Patent Laid-Open No. 60-12213 in which the amount of the edge drop of the steel strip on the final stand outlet side in the above-described tandem cold rolling mill stands is measured, the measured amount of the edge drop and a target amount of the edge drop are subjected to a comparison, and accordingly controlling the widthwise directional amount of shift of the work roll having the above-described tapered portion and the roll bender pressure.
In the above-described method in which the edge drop is controlled by shifting the single-end-tapered work rolls, the vertically-disposed work rolls are shifted by the same distance in the opposing directions. Therefore, in a case where the amounts of the edge drop of the steel strip generated due to the hot rolling operation are different from each other between the two widthwise end portions, the edge drop on both sides may not be corrected because the same amount of the edge drop cannot be obtained at the widthwise end portions by shifting the single-end-tapered work roll in accordance with one amount of the edge drop measured by the edge drop gauge with a target amount of the edge drop. Even worse, is a case where the amounts of the edge drop generated by the hot rolling operation are considerably different from each other between the two widthwise end portions. The problem cannot be overcome by the conventional method. The conventional method may result in a great amount of the edge drop being generated at one end portion and the occurrence of edge-up at another end portion. Furthermore, if the vertically disposed single-end-tapered work rolls are simultaneously shifted in the same direction, a contraction and zigzag movement may take place on the mill inlet side because the steel strip tends to move in the direction of the work rolls. For this reason, the conventional method of correcting edge drop has not satisfactorily overcome the above-described problems.
In the case where the edge of the steel strip is not cut in the pickling process, the width deviation generated in the hot rolling operation is undesirably maintained.
The position at which the amount of the edge drop is measured for use in the above-described cold rolling process, defined by the distance from the widthwise end portion in the cold rolling process, and the position at which the amount of the edge drop is detected, defined by the distance from the widthwise end of the final product, are different from each other due to the above-described width deviation when the final product is obtained by cutting the edge after the cold rolling process. As a consequence, a product displaying equal edge drop in its lengthwise direction cannot always be obtained even if the above-described control is performed in the cold rolling process.
Furthermore, since the width deviation affects the occurrence of the edge drop in the cold rolling process, the change in the edge drop in the cold rolling process becomes too large even if the above-described control is performed in the cold rolling process.
When the number of the stands on which the rolling mills each having a single-end-tapered work roll are mounted is not sufficiently large, it takes time for the portion of the steel strip to be controlled to be conveyed to the final stand outlet side. The delay exists even if the amount of shift of the single-end-tapered work roll is changed for the purpose of reducing the edge drop in the subject rolling mill. The conventional method cannot cause a satisfactory response if the disturbance is changed such that the thickness distribution or acceleration/deceleration of the rolling mill which affects the generation of the edge drop at the time of the cold rolling process is changed. Therefore, the amount of the edge drop cannot be maintained at a constant value in the lengthwise direction of the steel strip. As a result, the manufactured product may display edge drop or edge up at the side ends of the steel strip.